Arya Stark started her incredibly popular kill list very early on in Game of Thrones.
After escaping King's Landing when King Joffrey beheaded her father, the youngest Stark daughter wielded her sword Needle at anyone who stood in the way of Winterfell.
Having learned to sword fight by Syrio Forel and then trained to kill far more effectively by the Faceless Men, Arya is a force to be reckoned with. Even a certain supernatural entity couldn't stop her, and that really is saying something.
Currently on her way back to King's Landing with the Hound at time of writing, Arya is clearly not done waging her personal war against those who need to pay for their injustices.
Her oft-talked about list was regularly brought up in more "relaxed" moments across the show's seasons, so we can expect it to crop up once again. All that said though, who still lives... and who's been stuck with the pointy end?